The present invention is related to improvements in valve metal capacitors and an improved method of forming valve metal capacitors. More specifically, the present invention is related to methods for manufacturing valve metal capacitors which provides an improvement in volumetric efficiency while maintaining or improving electrical performance, more specifically equivalent series resistance, ESR.
It is standard practice in the manufacture of valve metal capacitors, particularly surface mount valve metal capacitors, to form a monolithic structure comprising an anode wire extending from the anode core wherein a dielectric and charge collecting cathode is on the surface of the monolith with a dielectric between the anode core and cathode. The manufacturing process includes attaching the anode wire to a lead frame at a first location and attaching the cathode to a lead frame at a second location.
FIG. 1a schematically illustrates a typical surface mount valve metal capacitor, 1, in cross-sectional view. In FIG. 1, the valve metal anode core, 2, has an anode wire, 3, extending there from. The anode wire is electrically connected to an anode lead, 4, typically by welding. A dielectric, 5, on at least a portion of the anode separates the anode from a cathode, 6. The cathode is electrically connected to a cathode lead, 7, by conductive adhesive, 11, and the entire structure, except for the contact portions of the anode lead and cathode lead, is encased in a non-conducting material, 8. The anode lead, 4, and cathode lead, 7, originates as a portion of a near-continuous lead frame in the form of an array, as is well documented in the art, with multiple anode leads and cathode leads integral to a common lead frame. The anode and cathode leads are electrically disconnected when capacitors are singulated such as by dicing.
In the case of aluminum based valve metal surface mount capacitors, it is standard practice to utilize aluminum in the form of foil for the anode, with roughened surfaces to increase surface area per unit volume, anodized to form a dielectric layer on the base material, and shaped into element(s) similar in size to the length and width of the overall device. The reason for employing aluminum in the form of relatively thin foil is that the preferred method to roughen the surfaces is only capable of reaching relatively shallow depths, so the most volumetrically efficient form is a thin sheet or foil. Each anodized element is processed to create a conductive polymer layer, acting as a primary cathode, over a portion of the anodized element thereby creating an active area of capacitance. Additional cathodic layers are added to protect the primary cathode, to collect the electrical current from the active area, and to conduct the electrical current to a leadframe extension that is located within the device encapsulant. Also within the encapsulant space, a portion of the anodic element is preserved without cathodic layers, serving as the anode extension, where this portion is attached to a separate leadframe extension. Both anode and cathode extensions of leadframe extend beyond the molded encapsulant and subsequently processed to form device leads which provide a means for the device to be soldered to printed circuit board mounting pads. The individual capacitive element is much thinner than the space allowed in typical industry standard surface mount capacitor package, thus additional capacitance is gained by stacking multiple individual capacitive elements on one another within the encapsulated device. Additional capacitive elements are stacked on top of the first element and attached to the first element by the same means as the first element is attached to the leadframe. The configuration of the first and/or subsequent elements may also be configured on the opposite side of the respective singular leadframe extensions.
FIG. 1B schematically illustrates a typical aluminum based surface mount valve metal capacitor, 100, in cross sectional view. In FIG. 1B, there are multiple capacitive elements, 101, that are constructed from an aluminum foil, 102, that has been processed to make its surfaces porous, 103. Over its entire surface, or just a portion of its surface, the porous aluminum foil is anodized to create a dielectric layer, 104. The foil is then processed to create a conductive polymer layer, 105, over the dielectric layer. A protective carbon layer, 106, is then applied over the conductive polymer layer, and a silver paint layer or metal filled organic resin layer, 107, is applied over the carbon layer to provide a capability to collect the electrical current from the capacitive region of the anode element. There is also a region, 108, of the anode element that is preserved from any of the capacitive associated layers, which acts as an anode extension and serves as the conductive path for the anodic electrical current. Thus deriving a complete capacitive element, 101. These capacitive elements are attached to a leadframe structure, 110, where the cathodic portion of the capacitive element is adhered to the cathodic portion of the leadframe, 111, with silver adhesive, 112, and the anode extension, 108, is attached to the anodic portion of the leadframe, 113, by welding. Additional capacitive elements are stacked on top of other capacitive elements and attached by the same methods as the first capacitive element(s) are to the leadframe extensions. This structure is then encapsulated in a thermosetting resin, 114. The leadframe extensions are processed to exist at the bottom surface of the encapsulant, 115, providing suitable mounting surfaces for soldering to a printed circuit board.
It is necessary for the anodic components and the cathodic components to be electrically separated as would be readily understood. This requirement creates a loss in volumetric efficiency since a significant volume of the ultimate capacitor does not contribute to capacitance. For example, with reference to FIG. 1, that portion of the capacitor surrounding the anode lead wire and anode lead provides no electrical purpose except to attach the lead frame to the lead wire with sufficient separation from the cathode layers. This problem is exacerbated by the necessity to provide enough separation between the active area of the capacitive element and the weld point, 9, in order to ensure that the effects of the weld operation, radiating unabated through the environment towards the sensitive and unprotected dielectric and cathodic layers, do not degrade the quality and performance of those layers. Shielding of the element from the weld process does not prove beneficial to reducing the occupied volume because practical limits of manufacturing precision prevent shortening of the distance required beyond that required without shielding. When multiple capacitive elements are combined into one capacitor the volumetric efficiency is even further eroded.
Yet another issue is the thickness of the current collecting cathodic layers which are typically some combination of conductive polymer layers and/or metal filled layers. These layers must have sufficient thickness to conduct a suitable amount of electrical current along the length and breadth of the capacitive element to the leadframe to achieve an acceptably low equivalent series resistance (ESR). In the case of solid electrolytic aluminum capacitors of typical construction containing stacked capacitive elements, the cathodic layers have the additional burden of carrying all of the electrical current for all capacitive layers stacked upon them away from the leadframe.
Electronic device manufacturers, who are the primary purchasers of surface mount capacitors, have a large installed manufacturing base tailored to mounting a surface mount capacitor onto a circuit board, or related element, to form an electrical sub assembly. Therefore, it is a necessity to provide capacitors which are structurally similar to surface mount capacitors as discussed above. Particular regards is necessary for the size, shape, and dimensions of the device and for the size, shape, and dimensions of the attachment locations. Unfortunately, the electronics industry is also constantly seeking to miniaturize electronic devices, or extract greater capacity and capability from the same size devices. This forces the manufacturer of components, such as capacitors, to seek more functionality in a given volume of space. These contradictory requirements have lead to the desire to provide a surface mount capacitor which has a higher volumetric efficiency or capacitance per unit volume while mimicking an industry standard surface mount capacitor in size and lead orientation. To address the loss in volumetric efficiency due to the anode attachment to its respective leadframe of typical valve metal capacitors, some manufacturers have attempted to locate the attachment outside of the device encapsulant. Some methods of connecting an anode extension to a preexisting external terminal, external to an encapsulant, have been proposed in U.S. Pat. Nos. 6,819,546 and 7,161,797. These methods involve forming a portion of the traditional leadframe material, or equivalent, embedded in the encapsulant, or as part of the overall encapsulating shell, and connecting the edge of the terminal to the exposed anode extension with a conductive layer applied onto the end of the device.
Other methods of similar construction are shown in US Application 2010/0165547. This application describes a device where the anode extension, and a portion of the applied conductive cathode, is exposed outside of the protective encapsulant. The end surfaces of the device from which the anode extension(s) and cathode layer is exposed are then flame sprayed, and subsequently made solderable, to create a terminal on each end of the device. This applied terminal material exists only on the end faces of the device, and does not have significant presence on the bottom, or surface mounting surface, of the device. It also covers the entire end faces of the device. This design represents a valve metal device with the terminal structure of an MLCC device. These terminal configurations are undesirable to customers, as these devices are not interchangeable with the industry standard termination specifications for valve metal capacitors. Further, these terminal configurations are undesirable to customers because the terminals extend the full width of the device. Per industry standard, the mounting pad on the printed circuit board is always more narrow than the device terminal due to the required stabilizing effect on the device during the soldering process of mounting the device to the PCB. Thus, when the terminal extends the full width of the device, the mounting pad on the printed circuit board is wider than the device, effectively requiring more space on the circuit board than can ever be filled by the capacitive device with this terminal configuration, resulting in less than ideal volumetric efficiency. Thus, a device that has terminals that are significantly narrower than the width of the device requires mounting pads on the printed circuit board that are narrower than the capacitive device, and thus requiring less space on the PCB, resulting in greater volumetric efficiency. It is preferred that a device would conform to the industry standard and preferably the device terminal would be 0.4 mm, or more, narrower than the device case. Typical construction methods of solid electrolytic valve metal capacitors as described here, which utilize a leadframe to terminate the device, meet such desirable terminal configuration as just described. Still further, terminal configurations in which the terminal reaches the top surface of the device, as those disclosed in U.S. Pat. Nos. 6,819,546 and 7,161,797, and US Application 2010/0165547, is also undesirable. This is due to a common condition of modern electronic devices exhibiting RF transmission, or those sensitive to external RF and EM interference, as in cellular telephones, where conductive metal grounded shielding is placed over the circuit board to mitigate such problems. In these devices, the shielding can come into contact with the top of the devices mounted to the PCB. Thus, devices with terminals reaching the top of the device would provide an electrical path between such terminals and the grounded shield, rendering the device and the circuit inoperable. Due to the conditions described above, it would be desirable to have a device that has improved volumetric efficiency potentially gained by externally attaching the anode extension to the terminal, while maintaining the exact terminal configuration of industry standard valve metal surface mount capacitors. Other reasons for providing a terminal with industry standard configuration are the customer's desire for rework of product. Many customers prefer that after the capacitor is soldered to the PCB a fillet of solder material is visible between the terminal and the pad. This allows for the device to be more easily removed if desired by the customer. As described in U.S. Pat. Nos. 6,819,546 and 7,161,797 the device end has a layer that provides connection from the anode to the terminal embedded in the encapsulate. Two potential problems arise from this in regards to the fore mentioned rework. One such problem is the careful attention needed to insure the connecting layer is solderable. Many described features may result in a surface that is not solderable, though is electrically connective. Another such problem related to rework is the mechanically integrity of the connective layer. If rework is needed the heat used to unmount the device may damage the connection layer and ultimately damage the device. It would be preferable to have a device with a terminal that is solid metal on all exposed surfaces so as to prevent damage to the device during rework processes and provide a continuous surface between the pad surface and end portions of the device to form a solder fillet.
Other methods of constructing surface mount solid electrolytic capacitors have been proposed such as those found in U.S. Pat. No. 6,185,091. These teachings still lead to volumetric inefficiencies. The focus is a construction with performance improvement related to its impact on an electrical circuit. The design requires the attachment of anode and cathode extensions. These teachings describe terminals that are mechanically attached prior to encapsulation. As described above this occupies space inside the encapsulation that lowers the volumetric efficiency of the device. In addition, no methods of attachment are taught in this patent and must be assumed to follow conventional methods of attachment that have no advantage in volumetric efficiency.
The volumetric efficiency of devices such as those disclosed in U.S. Pat. Nos. 6,819,546 and 7,161,797, and US Application 2010/0165547 is also severely limited due to the cathode layer construction which is specified as silver paste which is commonly used in the industry. There is a significant negative impact on volumetric efficiency by using silver paste as a current collecting cathode layer due to the relatively low conductivity of the paste, compared to solid metal conductor, which forces the use of a significant thickness of paste in order to conduct the necessary current to achieve the ESR performance expected of these products.
The present invention provides a capacitor which eliminates the problems in the art.